CALCULATION OF SOLAR-INDUCED FLUORESCENCE IN SURFACE AND SUBSURFACE WATERS

Solar-induced chlorophyll a fluorescence can be observed in upwelling irradiance spectra, but two major corrections are necessary before the signal may be used to detect phytoplankton pigment or primary product ion. A two-component correction model was developed for use in both su rface and subsurface waters. The first component separates upwelling i rradiance resulting from fluorescence from total upwelling irradiance using a simple linear approximation between two wavelengths on either side of the chlorophyll a emission spectrum. Fluorescence accounted fo r 10 to 40% of total upwelled irradiance at the surface and up to 80% in subsurface waters. The second component corrects for attenuation of irradiance between the phytoplankton source and the detector using at tenuation terms that incorporate the angular distributions of solar ex citation and isotropic fluorescence emission irradiances. Results from our model compare favorably with models rigorously derived for surfac e application, and - because it is not based on the single-scattering assumption, - our model also can be used in subsurface waters. The div erse water masses sampled (fjord, coastal, and open ocean) belonged to a single optical type, according to a classification using K-d(490) a nd a(phy)(490) (rather than chlorophyll a); these results indicate tha t our model can be used in diverse environments to extract a corrected fluorescence signal from upwelling irradiance spectra.